The invention described herein pertains generally to organic gels and more specifically to melamine-formaldehyde (MF) aerogels and methods for their preparation.
Aerogels are a unique class of ultrafine cell size, low density, open-cell foams. Aerogels have continuous porosity and a microstructure composed of interconnected colloidal-like particles or polymeric chains with characteristic diameters of 100 A (angstrom). The microstructure of aerogels is responsible for their unusual acoustic, mechanical, optical and thermal properties. [Fricke, Sci. Am., 285(5): 92 (1988); Fricke, in Sol-Gel Science and Technology (Aegerter et al., eds.) (World Scientific Publishing, N.J.) 482 (1989).] The microstructure imparts high surface areas to aerogels, for example, from about 350 m.sup.2 /g to about 1000 m.sup.2 /g. Their ultrafine cell/pore size minimizes light scattering in the visible spectrum, and thus, aerogels can be prepared as transparent, porous solids. Further, the high porosity of aerogels makes them excellent insulators with their thermal conductivity being approximately 100 times lower than that of the fully dense matrix. Still further, the aerogel skeleton provides for the low sound velocities observed in aerogels. [Aerogels (Fricke ed.) (Springer-Verlag N.Y. 1988).]
As a result of their high porosity, aerogels exhibit elastic moduli many orders of magnitude smaller than their full density analogs. A simple scaling law relates the aerogel modulus to its density (.rho.), that is, E=c.rho..sup.n. The scaling constant, n, and prefactor, c, are sensitive to variations in the aerogel microstructure.
Traditional aerogels are inorganic (for example, silica, alumina or zirconia aerogels), made via the hydrolysis and condensation of metal alkoxides, for example, tetramethoxy silane [Teichner et al., Adv. Coll. Interf. Sci., 5: 245 (1976); Brinker et al., J. Non-Cryst. Solids, 48: 47 (1982); J. Non-Cryst. Solids, 63: 45 (1984)].
Recently, organic aerogels from the sol-gel polymerization of resorcinol (1,3 dihydroxy benzene) with formaldehyde under alkaline conditions have been developed as disclosed in U.S. Pat. No. 4,873,218, issued Oct. 10, 1989, to Richard W. Pekala. [Pekala et al., J. de Physique. Colloque Suppl., 50(4): (4-33) (1989); Pekala, J. Mat. Sci., 24: 3221 (1989); Pekala and Kong, Polym. Prpts., 30(1): 221 (1989); and Pekala and Stone, Polym. Prpts., 29(1): 204 (1988).]
Although the resorcinol-formaldehyde aerogels (RF aerogels) exhibit minimal light scattering, they are dark red in color and have a large absorption coefficient within the visible spectrum. The color centers present in the RF aerogels result from oxidation products (for example, quinones) formed during the polymerization. Their presence has limited the use of the RF aerogels for certain optical applications where the material needs to transmit light and be essentially colorless, that is, non-absorptive in the visible spectrum.
The present invention overcomes the optical limitations of RF aerogels by providing organic aerogels of low density and high surface area, produced by the sol-gel polymerization of melamine with formaldehyde; such aerogels are not only transparent, but also essentially colorless having a slightly bluish tinge.
The MF aerogels are prepared by the aqueous, sol-gel polymerization of melamine (2,4,6 triamino s-triazine) with formaldehyde followed by supercritical extraction. Described herein are processes for preparing MF aerogels which processes are different from those used to prepare RF aerogels, primarily in that acidic conditions are necessary to promote condensation of intermediates in the polymerization process leading to gel formation. Synthetic conditions, for example, reaction time and pH, affect the density, transparency and microstructure of the resultant MF aerogels. Representative densities of the MF aerogels are low from about 100 mg/cc to about 800 mg/cc, preferably from about 100 mg/cc to about 750 mg/cc; and the surface area is high, for example, about 1000 m.sup.2 /g.
Kistler described organic foams prepared from nitrocellulose, cellulose, agar and egg albumin using a supercritical drying procedure. [Nature, 127: 741 (1931).]
Examples of commercially available "low-density" materials are plastic "blown cell" foams, such as, polyurethane cushions and polystyrene coffee cups. Asymmetric membranes and filters, on the other hand, are representative of commercially available "microcellular" materials. The processes used to make such products are generally not suitable for making aerogels, however, because they are limited by a trade off between density and cell size. That is, such processes produce relatively low density products only at the expense of increased cell size, or produce products having small cell size at the expense of those products having increased density. Aerogels, on the other hand, have both low density and small cell size, as well as meeting other requirements of various applications (for example, composition, homogeneity, size and strength).
Differentiated from the organic aerogels, such as, RF and MF aerogels, are the relatively macrocellular (having large cell sizes) foamed organic polymers and organic foam composite materials that are well-known and used in the insulation, construction and similar industries. Such foams are not generally suitable for applications where both very low density and ultrafine cell sizes are needed, such as in many high-energy physics applications, or as parts for inertial confinement fusion targets. A requirement for such organic materials is not only very low density, but generally at least over an order of magnitude smaller cell size than foams produced using other conventional techniques such as the expansion of polymer/blowing agent mixtures, phase-separation of polymer solutions and replication of sacrificial substrates, to name a few. Some of such prior art methods have produced phenol-formaldehyde and phenol-urea foams, but again, such foams have a compact cellular structure, but not the sufficiently small cell sizes necessary for high-energy physics applications.
Such materials do not exhibit the desired low density, combined with the ultrafine cell structure characteristic of aerogels, and are thus not suitable for applications in high energy physics or as parts for inertial confinement fusion targets. The current production of low density materials with ultrafine pore sizes (less than or equal to 1000 A) has largely been limited to aerogel technology, particularly to silica aerogels.
Silica aerogels are being developed as superinsulating material for double pane windows. Organic aerogels would be expected to have an even lower thermal conductivity and, thus, provide less heat loss in insulating applications.
The presence of silicon, having an atomic number (Z) of 14, in silica aerogel systems often limits its effectiveness for many applications, such as in high energy physics or as parts for inertial confinement fusion targets and the like, where a low number for Z (atomic number) is preferred. Pure organic foams or aerogels, consisting of mostly carbon (Z=6), and hydrogen (Z=1) with some oxygen (Z=8), are suitable for such applications. The organic composition of MF aerogels provides them with a low average atomic number, making them ideal candidates for high energy physics applications and as parts for inertial confinement fusion targets.
Other potential applications for the MF aerogels of this invention include, but are not limited to, uses as catalyst supports, permselective membranes, thermal insulators, gas filters in chemical processing chromatographic packings, sensors, lenses, solar collectors and impedance matching devices. Future applications could include lightweight insulative clothing, fire-retardant architectural materials, high resolution sonic detectors, autofocus cameras, dielectric spacer material for electronics and magnetics, acoustic and thermal absorbers for packaging valuable temperature-sensitive products.
Accordingly, it is an object of the present invention to provide a low density organic aerogel which exhibits continuous porosity and ultrafine cell size and is not only transparent, but also essentially colorless, that is, non-absorptive in the visible spectrum.
Another object of the invention is to provide a new synthetic route for the production of organic aerogels. The aqueous, sol-gel polymerization of melamine with formaldehyde requiring a pH change, followed by supercritical extraction, lead to the formation of a new type of organic aerogel. Low densities (from about 0.1 to about 0.8 g/cc), high surface areas (about 1000 m.sup.2 /g) and optical clarity are only a few of the characteristics of the MF aerogels of this invention.
Additional objects, advantages and novel features of the invention, together with additional features contributing thereto and advantages accruing therefrom will be apparent from the following description and the accompanying illustration of one or more embodiments of the invention and the description of the preparation techniques therefor, as described hereinafter. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.